Semiconductor wafers are frequently cleaned with a cleaning solution into which megasonic energy is propagated. Megasonic energy cleaning apparatuses typically comprise a transducer assembly which is a piezoelectric transducer coupled to a transmitter. The transducer is electrically excited such that it vibrates, and the transmitter transmits high frequency energy into a liquid layer coupled with the wafer. The agitation of the cleaning fluid produced by the megasonic energy loosens particles on the semiconductor wafers.
The number of watts applied to the transducer is one way to measure the acoustical energy applied to the wafer. The problem with measuring the acoustical energy in this fashion is that all energy applied to the transducer does not translate directly into the actual strength of acoustical energy applied to the substrate. Some of the applied power provided to the transducer is lost as heat, some of the energy is reflected, while some of the energy is lost as mechanical energy not applied in the direction of the substrate. Other factors that play into the loss of energy are differences in the characteristics of individual transducers stemming from the manufacture and assembly of the individual components, the differences in frequency characteristics that occur in each unit, and the operating temperature of the transducer assembly and it's components.
Therefore, there remains a need to accurately measure the acoustical energy applied to a substrate that takes into account the factors discussed above.